Solid-state rearrangement of sandwich-type polyoxometalate-dopamine nanohybrid to the nanoflower Keggin polyoxometalate: synthesis, characterization, and catalytic efficiency

Article abstract of DOI:10.1080/00958972.2020.1753713

Sponge-like inorganic-organic nanohybrid consisting of sandwich-type polyoxometalate of [P₂W₁₈Ce₃(H₂O)₂O₇₁]^12? (P₂W₁₈Ce₃) and dopamine (DA) was fabricated by a simple procedure. The DA/P₂W₁₈Ce₃ sponge-like nanohybrid is turned into a nanoflower by calcination at 300 °C. The results showed a structure rearrangement of sandwich-type polyoxometalate to the Keggin-type polyoxometalate along with this morphology change. The nanomaterials were fully characterized by scanning electron microscopy (FE-SEM), energy dispersive X-ray analysis (EDX), thermogravimetric analysis (TGA), Fourier transform infrared (FT-IR) spectroscopy, texture analysis (BET), and X-ray powder diffraction (XPRD). The DA/P₂W₁₈Ce₃ sponge-like nanohybrid and the Keggin nanoflower showed efficiency toward degradation of methylene blue (MB) and rhodamine B (RhB). Moreover, the catalytic activity of the Keggin nanoflower was examined in the oxidation of sulfides to sulfones with H₂O₂ as oxidant in water at room temperature. The catalyst was reused at least four times without loss of its catalytic activity.

Full text of DOI:10.1080/00958972.2020.1753713

Journal of Coordination Chemistry
ISSN: 0095-8972 (Print) 1029-0389 (Online) Journal homepage: https://www.tandfonline.com/loi/gcoo20
Solid-state rearrangement of sandwich-type
polyoxometalate-dopamine nanohybrid to the
nanoflower Keggin polyoxometalate: synthesis,
characterization, and catalytic efficiency
Soleiman Fazeli Pirdosti, Roushan Khoshnavazi & Elham Naseri
To cite this article: Soleiman Fazeli Pirdosti, Roushan Khoshnavazi & Elham Naseri (2020):
Solid-state rearrangement of sandwich-type polyoxometalate-dopamine nanohybrid to the
nanoflower Keggin polyoxometalate: synthesis, characterization, and catalytic efficiency, Journal of
Coordination Chemistry, DOI: 10.1080/00958972.2020.1753713
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JOURNAL OF COORDINATION CHEMISTRY
https://doi.org/10.1080/00958972.2020.1753713
Solid-state rearrangement of sandwich-type
polyoxometalate-dopamine nanohybrid to the
nanoflower Keggin polyoxometalate: synthesis,
characterization, and catalytic efficiency
Soleiman Fazeli Pirdosti, Roushan Khoshnavazi and Elham Naseri
Department of Chemistry, University of Kurdistan, Sanandaj, Iran
ABSTRACT
ARTICLE HISTORY
Sponge-like inorganic-organic nanohybrid consisting of sandwich-
Received 28 November 2019
Accepted 30 March 2020
12ꢀ
type polyoxometalate of [P
2
W18Ce
3
(H
2
O)
2
O
71
]
2 3
(P W18Ce ) and
dopamine (DA) was fabricated by a simple procedure. The DA/
KEYWORDS
P W
2
18Ce
3
sponge-like nanohybrid is turned into a nanoflower by
ꢁ
Polyoxometalates; dopa-
mine; nanohybrid; synthetic
dyes; oxidation of sulfides
calcination at 300 C. The results showed a structure rearrangement
of sandwich-type polyoxometalate to the Keggin-type polyoxome-
talate along with this morphology change. The nanomaterials were
fully characterized by scanning electron microscopy (FE-SEM),
energy dispersive X-ray analysis (EDX), thermogravimetric analysis
(
TGA), Fourier transform infrared (FT-IR) spectroscopy, texture ana-
lysis (BET), and X-ray powder diffraction (XPRD). The DA/P W18Ce
2 3
sponge-like nanohybrid and the Keggin nanoflower showed effi-
ciency toward degradation of methylene blue (MB) and rhodamine
B (RhB). Moreover, the catalytic activity of the Keggin nanoflower
was examined in the oxidation of sulfides to sulfones with H O as
2
2
oxidant in water at room temperature. The catalyst was reused at
least four times without loss of its catalytic activity.
1. Introduction
Polyoxometalates (POMs), also known as polyanions, are an important class of com-
pounds formed from metal-oxygen building blocks. The common building metals are
CONTACT Roushan Khoshnavazi
r.khoshnavazi@uok.ac.ir
ß 2020 Informa UK Limited, trading as Taylor & Francis Group

2
S. F. PIRDOSTI ET AL.
the early transition metal elements, tungsten(VI), molybdenum(VI), vanadium(V),
niobium(V), and tantalum(V) [1–3]. The diversities in composition and structure are special
features of polyoxometalates. These features make POMs attractive for designing of
Br o€ nsted acid and redox homogeneous and heterogeneous catalysts. The catalytic proper-
ties of polyoxometalates are controlled by both their structure and composition [4–7].
Structures are changed by influence of the conditions such as temperature or counter
ions, an important feature of POMs [8–11]. Catalytic applications of POMs are limited
because of (1) POMs in bulk forms are usually non-porous solids with low surface area
and (2) high solubility of POMs in polar solvent which disfavors their recovery and reus-
ability. Attempts to resolve such problems mainly have been focused on the synthesis of
POM nanoparticles (NPs). POM nanoparticles can be obtained by dispersion of POM’s
onto nano-sized supports such as silica, carbon, zirconia, alumina or porous silica [12–18].
Inorganic-organic nanohybrids as an important class of the POM nanoparticles can be fab-
ricated by simple ionic self-assembly between suitable organic molecules and POM anions
[3,19,20]. This method is simple and provides a good control on sizes and shapes in fabri-
cation of hybrid nanomaterials. Dopamine (3,4-dihydroxyphenethylamine, DA) is an
organic chemical of the catecholamine and phenethylamine families that plays several
important roles in the brain and body. Recently, DA has been extensively employed in
surface modification, biomedicine, and Li-ion batteries [21–23].
Synthesis of 3-D flowerlike hybrid materials containing dopamine and phosphotungstic
9
ꢀ
acid was reported by Li et al. [24]. The Weakley-type EuW O
0 36
POM was employed to
1
assemble with the cationic DA into well-ordered nanoflower structures through simple
ionic self-assembly by Zhang et al. They demonstrated that the sizes and morphologies of
the 3-D nanostructures were dependent on the concentration and ratio of the anionic
and cationic components and catalytic performance was studied [18]. Herein, the sand-
wich-type POM of P W Ce was used to assemble with cationic DA for preparation of
2
18
3
hybrid nanomaterial through simple ionic self-assembly method. The structures, chemical
properties and catalytic efficiencies of the obtained nanomaterials were studied. The struc-
ture of P W Ce consists of three cerium(IV) ions sandwiched by two tri-lacunary Keggin
2
18
3
12ꢀ
moieties [25]. The structures of [P W Ce (H O) O ]
(P W Ce ) and the Keggin
2 18 3
2
18
3
2
2
71
3
ꢀ
[
PW O ] (PW ) polyoxometalate anions are shown in Figure 1.
12
40
12
2. Experimental
2.1. Materials and physical measurements
All chemicals and solvents were purchased from commercial sources and used without
purification. Doubly distilled water was used throughout. K (NH )H [P W Ce
9
4
2
2
18
3
(
H O) O ]ꢂ20H O (P W Ce ) was synthesized according to previously reported litera-
2
2
71
2
2
18
3
ture procedures [25]. The crystal structures of the synthesized nanohybrid and nano-
flower were characterized by an X’Pert Pro Panalytical, Holland diffractometer in 40 kV
and 30 mA with CuKa radiation (k ¼ 1.5418). Infrared spectra were recorded on a
Bruker Vector 22 FT-IR using KBr pellets. The morphologies of the samples were
revealed by a scanning electron microscope (FESEM-TESCAN MIRA3). The elements in the
samples were probed by energy dispersive X-ray (EDX) spectroscopy accessory to the
FESEM-TESCAN MIRA3 scanning electron microscopy. UV–vis absorption spectra were

JOURNAL OF COORDINATION CHEMISTRY
3
Figure 1. Ball-stick and polyhedral presentation of P W Ce (a) and (b) PW polyoxometalate
2
18
3
12
anions (b).
recorded on a single-beam UV–vis spectrometer (Spekol 1300). Thermogravimetric analy-
ses (TGA) were carried out using a STA PT-1000 LINSEIS. Nitrogen adsorption–desorption
isotherms were used to obtain the hybrid nanomaterial specific surface areas and pore
volumes were calculated by the Brunauer–Emmett–Teller (BET) method. The samples were
ꢁ
degassed at 100 C in vacuum overnight prior to adsorption measurements. The experi-
ꢁ
ments took place at ꢀ196 C under variable relative pressure. The equipment was a
ꢁ
Belsorp mini II adsorption analyzer (Bel-Japan) at ꢀ196 C. The acidity of samples was
probed by potentiometric titration. The sample (0.05 g) was suspended in acetonitrile and
stirred for 3 h. Then, the suspension was titrated with 0.05 M n-butylamine in acetonitrile
ꢀ
1
at 0.05 mL min . The electrode potential variation was measured with a WTW digital
nanolab model using a double-junction electrode.
2
.2. Synthesis of DA/P W Ce nanohybrid
2 18 3
In a typical experiment, the sample was prepared by adding P W Ce aqueous solu-
2
18
ꢀ1
3
ꢀ
1
tion (1 mL, 10 mg mL ) to DA aqueous solution (4 mL, 10 mg mL ) with stirring. The
solution became brown immediately but became turbid after 45 min. The precipitation
was completed after at least 6 days. The aqueous mixture was decanted and the
ꢁ
brown precipitate was collected, washed three times with water and dried at 40 C
overnight. We found that the synthesis procedure can be scaled up by ꢃ40.
2.3. Synthesis of PW12 nanoflower
The PW12 nanoflower was prepared by calcination of DA/P W Ce powder sample
2
18
3
ꢁ
(
500 mg in a porcelain crucible) at 300 C for 2 h in a laboratory electric furnace.

4
S. F. PIRDOSTI ET AL.
2.4. Catalytic activity
The catalytic activity and selectivity of the nanohybride was tested by measuring the
degradation of methylene blue (MB), rhodamine B (RhB) and methyl orange (MO) in
the presence of H O . The catalytic activity of the PW nanoflower also was tested in
2
2
12
the oxidation of sulfides to sulfones with H O2 as oxidant in water at room
2
temperature.
2.4.1. General procedure for the decomposition of dyes
In a typical process, 5 mg of the catalysts were dispersed into 20 mL of MB dye solu-
ꢀ
1
tion (10 mg L ), followed by addition of 100 mL of H O (30 wt. %) to the dye solution
2
2
under vigorous stirring. During the catalytic process, changes of the dye’s maximum
absorbance intensity were detected by using a UV–visible spectrophotometer. The dye
concentration was obtained from the calibration curve and the degradation efficiency
(
%) was calculated using 100 ꢃ (C ꢀ C)/C formula. The heterogeneous catalyst was
0
0
collected by centrifuge and washed with distilled water and recycled for the next run.
2.4.2. General procedure for the oxidation of sulfides
A mixture of the PW12 nanoflower (10 mg) as catalyst, 30% H O aqueous solution
2
2
(100 mL) and solvent (600 mL) was placed in a 10 mL glass bottle. After 5 min, the sub-
strate (1 mmol) was added under stirring. The reaction time was counted after the
addition of sulfide, and then the reaction mixture was stirred at the experiment tem-
perature for the appropriate time. The sample was collected from the mixture at time
intervals and then the progress of the reaction was followed by TLC (eluent: n-hex-
ane/EtOAc, 3:1) and stopped when complete conversion of the substrate was
observed. The catalyst was filtered off at the end of reactions, washed several times
ꢁ
with ethyl acetate followed by ethanol (4 ꢃ 5 mL), heated in an oven at 70 C over-
night and then reused using the same reaction conditions. The starting material and
product are insoluble in water and it was used just as an environment for stirring.
Therefore, the reaction mixture was transferred to a separating funnel and the product
was extracted with CH Cl (3 ꢃ 5 mL). After evaporation of organic layer, the crude
2
2
products were recrystallized from hot ethanol and the pure products were obtained in
94–98% yield. Recovering of the PW12 nanoflower catalyst was carried out in four con-
secutive experiments.
3
. Results and discussion
.1. FT-IR spectra
The FT-IR spectra of the nanohybrids and the precursor compounds of P W Ce poly-
3
2
18
3
oxometalate and the dopamine hydrochloride are compared in Figure 2. The FT-IR
spectrum of sponge-like nanohybrid (Figure 2A(a)) exhibits characteristic bands of
both the dopamine (Figure 2A(b)) and the sandwich polyoxometalate P W Ce
3
2
18
ꢀ
1
(
Figure 2A(c)). Bands at 1045, 1018, 947, and 773 cm observed in the spectrum of
DA/P W Ce were attributed to the P–O and W–O vibrations of the sandwich polyox-
2
18
3
þ
ometalate P W Ce [25]. The band corresponding to the NH in the P W Ce due
2
18
3
4
2
18
3

JOURNAL OF COORDINATION CHEMISTRY
5
Figure 2. FT-IR spectra of (A) DA/P W Ce (a) dopamine hydrochloride (b) and P W Ce (c) and
2
18
3
2
18
3
(
B) DA/P W Ce (a) PW nanoflower (b) and PW12 anion (c).
2 18 3 12
Figure 3. IR spectra of calcined DA/P W Ce at various temperatures.
2
18
3
to the DA replacement disappeared from the spectrum of DA/P W Ce . Meanwhile,
2
18
3
the primary peaks of DA were observed in the absorption bands of the sponge-
like nanohybrid.
3.2. Calcination
The experimental evidence of IR spectra and XRD patterns confirmed that the struc-
ture of P W Ce was intact upon treatment by DA. Figure 3 shows the IR spectra of
2
18
3
DA/P W Ce sample calcined at various temperatures. The guest water molecules in
2
18
3
ꢁ
the sponge-like nanohybrid are removed by calcination at 100 C without structure
ꢁ
decomposition. Heating at 200 C for 2 h caused some organic content to be removed
ꢁ
but P W Ce preserves its structure almost intact. Further heating (300 C for 2 h)
2
18
3

6
S. F. PIRDOSTI ET AL.
caused almost all organic content to be removed and moreover, a change was
observed in P W Ce structure. As an important point, the structural conversion does
2
18
3
not end to total P W Ce structure collapse. The IR (Figure 2B) spectra confirm con-
2
18
3
version of P W Ce to PW Keggin structure. The FT-IR spectrum (Figure 2B(b)) for
2
18
3
12
ꢁ
sample calcined at 300 C shows four stretching vibrations at 827, 903, 984, and
ꢀ
1
1
082 cm , consistent to those observed for PW12 Keggin structure (Figure 2B(c)). The
observed bands correspond to the stretching vibrations ꢀas (W–O –W), ꢀ (W–O –W),
c
as
b
ꢀas (W–O ), and ꢀ (P–O ) of PW Keggin structure, respectively [26]. The structure
d
as
a
12
ꢁ
further collapses by heating (400 C for 2 h) but a complete collapse took place under
ꢁ
heating at higher than 500 C (Figure 3).
3.3. TG analysis
The TGA curves of DA/P W Ce and the PW nanoflower sample are shown in Figure
2
18
3
12
4
1
. As shown in Figure 4(a), DA/P W Ce there is continuous decrease from 25 to
2 18 3
ꢁ
80 C with a weight loss of 9.75%, which is attributed to evaporation of lattice water
ꢁ
and incomplete decomposition of organic content. From 180 C a weight loss of
1
5.1% is observed, attributed to the decomposition of organic content and P W Ce
2 18 3
POM. For the PW12 nanoflower sample there are two obvious mass losses. The first
ꢁ
ꢁ
started at 25 C and ended at 170 C with a weight loss of 2.5%, which can be attrib-
ꢁ
uted to evaporation of water molecules adsorbed. The second loss observed at 370 C
with a weight loss of 9.0% corresponded to decomposition of H PW O [17, 27].
3
12 40
From the TGA curves, it can be concluded that PW12 nanoflower exhibits higher ther-
mal stability than DA/P W Ce nanohybrid.
2
18
3
3.4. X-ray powder diffraction
Figure 5 shows the XPRD patterns of DA/P W Ce and the PW nanoflower com-
2
18
3
12
pared with precursors. Figure 5A(a) shows the diffraction peaks of DA/P W Ce
3
2
18
include characteristic peaks of P W Ce sandwich polyoxometalate (Figure 5A(b)).
2
18
3
Figure 5B shows the XPRD patterns of H PW O in bulk form (Figure 5B(a)) and
3
12 40
PW12 nanoflower (Figure 5B(b)). The PW12 nanoflower shows pattern of typical
amorphous state, implying that the compound has less-ordered structures. However, it
can be observed that the XPRD pattern of PW12 nanoflower contains that of
H PW O sample [28].
3
12 40
3
.5. N adsorption–desorption isotherms
2
The N adsorption–desorption isotherms and pore size distributions of DA/P W Ce
2
2
18
3
and the PW12 nanoflower (77 K) are shown in Figure 6. The isotherms DA/P W Ce
2
18
3
(Figure 6(a)) and the calcined sample (Figure 6(b)) belonged to type II according to
IUPAC classification [29], which is a typical characteristic adsorption–desorption iso-
therm of mesoporous materials. From the distribution curves, the nanohybrids show
broad pore-size distributions of 1.2–30 nm (calculated by BJH method based on the
adsorption branch) and are centered at 2.20, 3.20, 2.00, 4.20, 6.20, 9.45, 12.50, and

JOURNAL OF COORDINATION CHEMISTRY
7
Figure 4. TGA curves of DA/P W Ce (a) PW nanoflower (b).
2
18
3
12
Figure 5. Powder X-ray diffraction patterns of (A) DA/P W Ce (a) P W Ce (b) and (B) Keggin
2
18
3
2
18
3
H PW O in bulk form (a) PW12 nanoflower (b).
3
12 40
2
3.00 nm (Figure 6, inset). The Brunauer–Emmett–Teller (BET) specific surface area, BJH
cumulative pore volume and BJH average pore diameter of DA/P W Ce nanoflowers
2
18
3
2
ꢀ1
3
ꢀ1
2
were 18.00 m g , 0.070 cm g , and 4.17 nm, respectively. The corresponding PW
12
ꢀ1
3
ꢀ1
nanoflower data were 10.46 m g , 0.055 cm g , and 7.13 nm, respectively. The
pores correspond to the voids separating the large primary particles. Thus calcination
affects the specific surface area, BJH cumulative pore volume, and BJH average pore
diameter of DA/P W Ce . Part may be related to the changing of size of primary par-
2
18
3
ticles but another part can be attributed to the blockage of pores by coke [29],
formed via decomposition of the organic content of DA/P W Ce . The sample is
2
18
3
black, thus coked, but not sufficiently for the specific surface area to be affected.
Regarding the pores, the calcined sample lost a part of the initial voids contained in
the initial DA/P W Ce and had considerably less marked hysteresis (Figure 6).
2
18
3

8
S. F. PIRDOSTI ET AL.
Figure 6. N adsorption-desorption isotherms at 77 K of DA/P W Ce (a) and PW nanoflower (b).
2
2
18
3
12
Inset shows BJH pore-size distribution plots of DA/P W Ce (a) and PW nanoflower (b).
2
18
3
12
Figure 7. SEM images of (A) DA/P W Ce and (B) PW nanoflower in different bars.
2
18
3
12
3.6. SEM images and EDS spectra
Figure 7A shows the SEM images of the DA/P W Ce nanohybrid in different bars.
2
18
3
The compound shows a sponge-like morphology decorated by 20–40 nm diameter
fibers. Figure 7B shows the SEM images of the PW12 nanoflower with different

JOURNAL OF COORDINATION CHEMISTRY
9
Figure 8. EDX spectra of (A) DA/P W Ce and (B) PW nanoflower.
2
18
3
12
magnifications. It reveals that the sample consists of similar flowerlike nanostructures
with an average diameter of about 3.0 lm and the petal thickness average about
47 nm. The petals covered by nanoparticle have an average diameter of about 19 nm.
Figure 8 shows the EDS images of the DA/P W Ce nanohybrid and the PW nano-
2
18
3
12
flower. The EDS analysis (Figure 8A) gave the elements of W, P, Ce, C, O, and N in the
samples, confirming the combination of P W Ce and DA. From EDX, potassium is
2
18
3
absent in the DA/P W Ce structure, thus, the electrostatic interaction is primarily
2
18
3
derived from oxygen of P W Ce and the protonated amine groups of DA.
2
18
3
3.7. Acid titration
The acidity of the PW12 nanoflower was probed by a potentiometric titration with n-
ꢁ
butyl amine (pK ¼ 10.60 at 20 C). The acidic environment around the electrode
a
membrane in terms of the measured potential difference is determined in this method
[20, 30]. The titration curve obtained is presented in Figure 9; the initial electrode
potential (E ) indicates the maximum strength of the acid sites while the value mmol
i
amine per g catalyst to reach the plateau indicates the total number of acid sites in
the titrated solid [30]. The acidic strength of the acid sites can be classified according
to the following ranges: E > 100 mV (very strong acid sites), 0 < E < 100 mV (strong
acid sites), ꢀ100 < E < 0 mV (weak acid sites) and E < ꢀ100 mV (very weak acid sites).
According to potentiometric titration curves, H PW O ꢂxH O presented very strong
3
12 40
2
acidic sites (E ¼ 572.2 mV). The PW nanoflower sample shows an E of 239.2 mV
i
12
i
which presented very strong acid sites. The total number of acid sites in the titrated
solid sample is about 0.25 mmol per g of the catalyst.
3.8. Catalytic properties
3.8.1. Catalytic properties of nanohybrids in degradation of dyes
The catalytic properties of DA/P W Ce nanohybrid and PW nanoflower in degrad-
2
18
3
12
ation of MB, RB, and MO dyes in the presence of H O at room temperature were
2
2
investigated. The control experiments show that dye degradation with H O in the
2
2

1
0
S. F. PIRDOSTI ET AL.
Figure 9. Potentiometric titration of PW12 nanoflower with n-butyl amine.
absence of catalyst is negligible, showing that H O alone is not effective in MB deg-
2
2
radation. The results represent that although both nanohybrids in presence of H O2
2
show high activity for MB and RB and MO degradation, the PW12 nanoflower shows
higher activities than DA/P W Ce nanohybrid. The activities for MB and RB are higher
2
18
3
than for MO. At a certain time interval of reaction, the changes of the dyes characteris-
tic peak intensities were checked until the solution become colorless.
In the presence of H O and DA/P W Ce nanohybrid or PW nanoflower, a 20 mL
2
2
2
18
3
12
ꢀ
1
solution of 10 mg L MB became colorless in less than 30 min. Under the same condi-
tions for RB, decolorization needs more than 90 min with PW12 nanoflower while DA/
P W Ce nanohybrid did not show decolorization. Decolorization of MO with DA/
2
18
3
P W Ce nanohybrid or PW nanoflower was not completed, but the latter was more
2
18
3
12
effective. The reusability and stability of the nanomaterials were investigated in decol-
ꢀ
1
orization of MB (5 mL, 10 mg L ). The results confirmed that DA/P W Ce nanohybrid
2
18
3
did at least three but PW12 nanoflower six reaction cycles. After each cycle, the com-
posite can be easily separated from the aqueous reaction system by using a centri-
fuge, washing with distilled water and using in the next cycle. However, the end time
of the reaction increased after the sixth cycle for PW12 nanoflower (Figure 10). The
reduction of the reaction rate in comparison with the fresh sample might be because
of the loss of catalyst during separation.
FT-IR spectra (Figure 11) showed no significant structural changes for the PW12
nanoflower catalyst after eight consecutive runs. The catalytic activities of DA/
P W Ce nanohybrid and PW12 nanoflower in the MB solution degradation were
2
18
3
tested at pHs of 2 and 9. The results confirmed that both catalysts show more catalytic
activities at pH 2. This can be ascribed to the nature of P W Ce sandwich and PW
12
2
18
3
Keggin polyoxometalates which are more stable in acidic media. The MB solution deg-
radation kinetics were investigated on PW12 nanoflower as catalyst. We found that the

JOURNAL OF COORDINATION CHEMISTRY
11
Figure 10. The reusability of DA/P W Ce (blue) and PW nanoflower (green) in decolorization
2
18
3
12
of MB.
Figure 11. The FT-IR spectra of fresh PW12 nanoflower (a) and after eight consecutive runs in
decolorization of MB (b) and after four consecutive runs in oxidation of sulfides (c).
ꢀ
1
first order kinetics (kobs, 0.064 min ; t1/2, 2.72 min) describe the degradation rate
of MB.
3.8.2. Catalytic properties of the PW12 nanoflower in oxidation of sulfides
Sulfoxides and sulfones are widely utilized as anti-bacterial, anti-fungal, anti-athero-
sclerotic, anti-ulcer, vasodilators and cardiotonic agents and as well as for activation of
enzymes. Traditional preparations with stoichiometric amounts of electrophilic
reagents such as peracids, dioxiranes, hypochlorites, periodates, and highly toxic oxo-
metal oxidants usually are accompanied by disadvantages such as long reaction times,
low yields, inconvenient reaction conditions, environmentally unfavorable catalysts
and the formation of toxic wastes [31,32].

1
2
S. F. PIRDOSTI ET AL.
Table 1. Selective oxidation of various sulfides to sulfones using H O catalyzed by PW nano-
2
2
12
flower.
a,b
Sulfone
Entry
1
Substrates
Time (min)
30
Isolated yield (%)
98
2
60
98
3
4
60
60
95
98
5
6
30
30
97
97
7
60
96
a
All the products are known and were characterized by FT-IR and by melting point comparison with those of
authentic samples.
Reaction conditions: sulfide (1 mmol), 35% H O (5.85 mmol), catalyst (50 mg), in H O, rt.
2 2 2
b
Various catalyst systems of organocatalysts, acid catalysts, enzymes, metal catalysts,
and organic-inorganic hybrid solid materials and recently, the polyoxometalates have
been used for oxidation of sulfides with H O as an oxygen source [33–35]. The oxida-
2
2
tion of methyl phenyl sulfide was selected as a model to find the optimum conditions.
The results showed that methyl phenyl sulfide (1 mmol) oxidation to methyl phenyl
sulfone was completed in distilled water, absolute ethanol and even in solvent-free
conditions with 10 mg catalyst and 100 mL H O after about 30 min. The oxidation
2
2
reaction of some different aliphatic and aromatic sulfides was examined with above
conditions and the results are presented in Table 1. The results showed that oxidation
of the sulfide substrates was catalyzed by PW12 nanoflower at room temperature and
short times. The results confirmed that for PW12 nanoflower at least four reaction
cycles could be used in oxidation of methyl phenyl sulfide. After each cycle, the com-
posite can be separated from the aqueous reaction system by using a centrifuge,

JOURNAL OF COORDINATION CHEMISTRY
13
washing with distilled water and using in the next cycle. As can be seen (Figure 11c),
the FT-IR spectra showed no significant structural changes for the PW12 nanoflower
catalyst after four consecutive runs.
4. Conclusion
A new inorganic-organic nanohybrid was synthesized by room temperature reaction
1
2ꢀ
of sandwich polyoxometalates of [P W Ce (H O) O ]
and dopamine hydrochlor-
2
18
3
2
2 71
3
ꢀ
ide. The nanohybrid was converted to a nanoflower composed of PW O
2 40
Keggin
1
ꢁ
structure by calcination at 300 C. The nanomaterials were structurally characterized
and their catalytic properties examined in decomposition of methylene blue (MB),
rhodamine blue (RB), methyl orange (MO) and oxidation of sulfides. The results show
that Keggin nanoflower has good catalytic performance.
Disclosure statement
No potential conflict of interest was reported by the author(s).
Funding
The authors would like to thank the University of Kurdistan Research Council for their support.
References
[1] M.T. Pope, A. Muller. Angew. Chem. Int. Ed. Engl., 30, 34 (1991).
[2] K. Zhu, J. Hu, X. She, J. Liu, Z. Nie, Y. Wang, C.H.F. Peden, J.H. Kwak. J. Am. Chem. Soc.,
1
31, 9715 (2009).
[
[
[
[
3] D.L. Long, E. Burkholder, L. Cronin. Chem. Soc. Rev., 36, 105 (2007).
4] L. Cronin, A. M u€ ller. Chem. Soc. Rev., 22, 7325 (2012).
5] S.S. Wang, G.Y. Yang. Chem. Rev., 115, 4893 (2015).
6] I.V. Kozhevnikov. Catalysts for Fine Chemical Synthesis. Catalysis by Polyoxometalates, Vol.
2, Wiley, Chichester (2002).
[
[
[
7] J. Macht, M.J. Janik, M. Neurock, E. Iglesia. Angew. Chem. Int. Ed., 46, 7864 (2007).
8] C.P. Pradeep, D.L. Long, L. Cronin. Dalton Trans., 39, 9443 (2010).
9] R. Khoshnavazi, H. Eshtiagh-Hossieni, M.H. Alizadeh, M.T. Pope. Polyhedron, 25, 1921
(
2006).
10] A. Teze, G. Herve. In Inorganic Syntheses, A.P. Ginsberg (Ed), Wiley, New York, pp. 27, 85
1990).
11] C. Comuzzi, G. Dolcetti, A. Trovarelli, F. Cavani, F. Trifir oꢀ , J. Llorca, R.G. Finke. Catal. Lett.,
6, 75 (1996). [InsertedFromOnline; ]
[
[
(
3
[
[
[
12] E. Rafiee, S. Eavani. RSC Adv., 6, 46433 (2016).
13] N. Narkhede, S. Singh, A. Patel. Green Chem., 17, 89 (2015).
14] S.M. Lauinger, J. Sumliner, Q. Yin, Z. Xu, G. Liang, E.N. Glass, T. Lian, C.L. Hill. Chem.
Mater., 27, 5886 (2015).
[15] A. Proust, B. Matt, R. Villanneau, G. Guillemot, P. Gouzerh, G. Izzet. Chem. Soc. Rev., 41,
7605 (2012).
[
[
16] X. Zheng, L. Zhang, J. Li, S. Luo, J.P. Cheng. Chem. Commun., 47, 12325 (2011).
17] T. Uma, M. Nogami. Chem. Mater., 19, 3604 (2007).

1
4
S. F. PIRDOSTI ET AL.
[18] H. Zhang, L.-Y. Guo, J. Jiao, X. Xin, D. Sun, S. Yuan. ACS Sustain. Chem. Eng., 5, 1358
(2017).
[
[
[
[
19] Z. Shi, C. Mei, G. Niu, Q. Han. J. Coord. Chem., 71, 1460 (2018).
20] A. Patel, K. Patel. J. Coord. Chem., 72, 3417 (2019).
21] M. Jiang, X. Zhai, M. Liu. J. Mater. Chem., 17, 193 (2007).
22] Z. Wang, J. Zhu, Y. Chen, K. Geng, N. Qian, L. Cheng, Z. Lu, Y. Pan, L. Guo, Y. Li, H. Gu.
RSC Adv., 4, 7483 (2014).
[23] R. Liu, X. Fu, C. Wang, G. Dawson. Chem. Eur. J., 22, 6071 (2016).
[24] H. Li, Y. Jia, A. Wang, W. Cui, H. Ma, X. Feng, J. Li. Chem. Eur. J., 20, 499 (2014).
[25] W.H. Knoth, P.J. Domaille, R.L. Harlow. Inorg. Chem., 25, 1577 (1986).
[26] C. Rocchiccioli-Deltcheff, M. Fournier, R. Franck, R. Thouvenot. Inorg. Chem., 22, 207
(1983).
[
[
27] T. Li, Q. Li, J. Yan, F. Li. Chem. Mater., 19, 2581 (2007). [InsertedFromOnline; ]
28] J. Javidi, M. Esmaeilpour, Z. Rahiminezhad, F. Nowroozi Dodeji. J. Clust. Sci., 25, 1511
(2014).
[29] J. Schnee, L. Fusaro, C. Aprile, E.M. Gaigneaux. Catal. Sci. Technol., 7, 6151 (2017).
[30] E. Rafiee, M. Joshaghani, S. Eavani, S. Rashidzadeh. Green Chem., 10, 982 (2008).
[31] C. Saux, C.L. Marchena, L.R. Pizzio, L.B. Pierella. J. Porous Mater., 23, 947 (2016).
[
InsertedFromOnline; ]
32] T.H. Chen, K.W. Kwong, N.F. Lee, D. Ranburger, R. Zhang. Inorg. Chim. Acta, 451, 65
2016).
[
(
[33] H. Haddadi, M.R. Farsani. J. Clust. Sci., 27, 373 (2016).
[34] M. Dabiri, M. Koohshari, F. Shafipour, M. Kasmaei, P. Salari, D. MaGee. J. Iran. Chem. Soc.,
1
3, 1265 (2016).
[
35] I.C. Santos, J.A. Gamelas, T.A. Duarte, M.M. Sim o~ es, M.G.P. Neves, J.A. Cavaleiro, A.M.
Cavaleiro. J. Mol. Catal. Chem., 426, 593 (2017).